/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.expression.spel.ast;

import org.springframework.asm.Label;
import org.springframework.asm.MethodVisitor;
import org.springframework.expression.EvaluationException;
import org.springframework.expression.TypedValue;
import org.springframework.expression.spel.CodeFlow;
import org.springframework.expression.spel.ExpressionState;
import org.springframework.util.Assert;
import org.springframework.util.ObjectUtils;
import org.springframework.util.StringUtils;

/**
 * Represents the elvis operator ?:. For an expression "a?:b" if a is not null, the value
 * of the expression is "a", if a is null then the value of the expression is "b".
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 3.0
 */
public class Elvis extends SpelNodeImpl {

    public Elvis(int pos, SpelNodeImpl... args) {
        super(pos, args);
    }


    /**
     * Evaluate the condition and if not null, return it.
     * If it is null, return the other value.
     *
     * @param state the expression state
     * @throws EvaluationException if the condition does not evaluate correctly
     *                             to a boolean or there is a problem executing the chosen alternative
     */
    @Override
    public TypedValue getValueInternal(ExpressionState state) throws EvaluationException {
        TypedValue value = this.children[0].getValueInternal(state);
        // If this check is changed, the generateCode method will need changing too
        if (!StringUtils.isEmpty(value.getValue())) {
            return value;
        } else {
            TypedValue result = this.children[1].getValueInternal(state);
            computeExitTypeDescriptor();
            return result;
        }
    }

    @Override
    public String toStringAST() {
        return getChild(0).toStringAST() + " ?: " + getChild(1).toStringAST();
    }

    @Override
    public boolean isCompilable() {
        SpelNodeImpl condition = this.children[0];
        SpelNodeImpl ifNullValue = this.children[1];
        return (condition.isCompilable() && ifNullValue.isCompilable() &&
                condition.exitTypeDescriptor != null && ifNullValue.exitTypeDescriptor != null);
    }

    @Override
    public void generateCode(MethodVisitor mv, CodeFlow cf) {
        // exit type descriptor can be null if both components are literal expressions
        computeExitTypeDescriptor();
        cf.enterCompilationScope();
        this.children[0].generateCode(mv, cf);
        String lastDesc = cf.lastDescriptor();
        Assert.state(lastDesc != null, "No last descriptor");
        CodeFlow.insertBoxIfNecessary(mv, lastDesc.charAt(0));
        cf.exitCompilationScope();
        Label elseTarget = new Label();
        Label endOfIf = new Label();
        mv.visitInsn(DUP);
        mv.visitJumpInsn(IFNULL, elseTarget);
        // Also check if empty string, as per the code in the interpreted version
        mv.visitInsn(DUP);
        mv.visitLdcInsn("");
        mv.visitInsn(SWAP);
        mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/String", "equals", "(Ljava/lang/Object;)Z", false);
        mv.visitJumpInsn(IFEQ, endOfIf);  // if not empty, drop through to elseTarget
        mv.visitLabel(elseTarget);
        mv.visitInsn(POP);
        cf.enterCompilationScope();
        this.children[1].generateCode(mv, cf);
        if (!CodeFlow.isPrimitive(this.exitTypeDescriptor)) {
            lastDesc = cf.lastDescriptor();
            Assert.state(lastDesc != null, "No last descriptor");
            CodeFlow.insertBoxIfNecessary(mv, lastDesc.charAt(0));
        }
        cf.exitCompilationScope();
        mv.visitLabel(endOfIf);
        cf.pushDescriptor(this.exitTypeDescriptor);
    }

    private void computeExitTypeDescriptor() {
        if (this.exitTypeDescriptor == null && this.children[0].exitTypeDescriptor != null &&
                this.children[1].exitTypeDescriptor != null) {
            String conditionDescriptor = this.children[0].exitTypeDescriptor;
            String ifNullValueDescriptor = this.children[1].exitTypeDescriptor;
            if (ObjectUtils.nullSafeEquals(conditionDescriptor, ifNullValueDescriptor)) {
                this.exitTypeDescriptor = conditionDescriptor;
            } else {
                // Use the easiest to compute common super type
                this.exitTypeDescriptor = "Ljava/lang/Object";
            }
        }
    }

}
